This invention relates to the separation of acidic gas constituents from gaseous mixtures containing the same in order to render such gaseous mixtures more suitable for their intended purposes.
Numerous instances are encountered of industrial processes employing a gaseous mixture for carrying out a reaction and of commercial applications involving the use of a gaseous mixture wherein it is necessary or at least highly desirable to remove or materially reduce the amount of any acidic gas constituent that may be present in such gaseous mixture. For example, in the production of ammonia from a synthesis gas containing hydrogen and nitrogen, it is essential that any carbon dioxide present in such synthesis gas as formed by substantially eliminated in order that the desired reaction can effectively take place. Similarly, any carbon dioxide, hydrogen sulfide, and/or carbonyl sulfide present in a natural gas or other gaseous hydrocarbon mixture must be significantly reduced in amount before such natural gas or gaseous hydrocarbon mixture is generally satisfactory for use as a fuel.
Such a gaseous mixture may be treated with an alkaline reagent to effect removal of its acidic gas content. Alkanolamines, especially the ethanolamines, have been widely used for this purpose and are conventionally employed in the form of their aqueous solutions. Absorption of the acidic gas constituent in such an aqueous alkanolamine solution is essentially by means of a strong chemical bond between the acidic gas and the alkanolamine; and for this reason it becomes possible to remove an acidic gas constituent to an extremely low level. On the other hand, a large amount of heat is required to break such strong chemical bond in order to regenerate the alkanolamine solution, with the result that the use of an alkanolamine for bulk removal of an acidic gas constituent from a gaseous mixture containing the same is rendered economically unattractive.
It has also been proposed to use a normally liquid dialkyl ether of a polyethylene glycol for this purpose. In this case absorption of the acidic gas constituent is almost wholly by means of a physical bond between the acidic gas and the polyethylene glycol dialkyl ether. Regeneration of such ether absorption medium is readily accomplished by stripping the same with an inert gas such as air, whereby bulk removal of an acidic gas constituent can be economically effected. Because of the limitation imposed by the equilibrium between the acidic gas in the gaseous mixture and that in the ether absorption medium, however, it is generally not possible to reduce the acidic gas content of such gaseous mixture to nearly as low a level as is frequently required. In addition, it is necessary to operate at a relatively high pressure in order to avoid too great a circulation rate of the ether absorption medium.
I have now found that, unexpectedly, these two absorption systems can be combined in such a way that bulk removal of an acidic gas constituent from a gaseous mixture containing the same and reduction of the amount of such acidic gas constituent to an extremely low level can be simultaneously effected in a very economical manner with avoidance to a large extent of the disadvantages inherent in each system alone.
U.S. Pat. No. 2,600,328 to Riensefeld et al. and U.S. Pat. No. 2,139,375 to Millar et al. are believed to be the most pertinent prior art. Each of Millar et al. and Riesenfeld et al. disclose removing acidic contaminants from gases by contacting said gases with an absorbent comprising a dialkyl ether of a polyalkylene glycol. Riesenfeld et al. additionally utilize an alkanolamine in conjunction therewith and Millar et al. indicate they may use an alkanolamine with their absorbent.
U.S. Pat. No. 2,600,328 discloses and I have verified that there is a strong tendency for phase separation of the absorbent to occur when removing one or more acidic gases from gaseous mixtures using a mixed absorbent comprising an ethanolamine and a dialkyl ether of a polyalkylene glycol. It is believed that such phase separation of the solvent is because of water-soluble salts formed in the solvent by reaction of the alkanolamine with the acid gases.
I have found that such phase separation of the solvent can be a principal limiting factor in the commercial use of mixtures of alkanolamines and dimethyl ethers of polyalkylene glycols for separation of acidic gas impurities from gaseous mixtures. Obviously, the formation of two liquid phases complicates the absorption process and requires a complex and expensive procedure for solvent recovery involving separation of the two phases and two stills or strippers for continuous recovery of the two liquid solvents. It is noteworthy that the prior art teaches away from the feasilibity of using an alkanolamine-polyalkylene glycol ether solvent as a single-phase solvent.